Several prominent Republican senators accused their GOP colleagues of “secretly” negotiating a sweeping immigration overhaul, urging them to open up about the details by next week.

The letter from Republicans on the Senate Judiciary Committee marked the latest flare-up in the GOP caucus over the immigration plan, which has not yet been introduced. Republican negotiators have urged their colleagues not to pre-judge the proposal, but others are getting frustrated by the lack of hard details and concern that majority Democrats will try to “rush” the bill through after it’s introduced.

“We believe it is critical that the public and the entire Senate body be given adequate time to read and analyze the contents of any immigration bill put forth by the majority,” the senators wrote in the letter Thursday to the four Republican negotiators: Florida Sen. Marco Rubio, Arizona Sen. John McCain, Arizona Sen. Jeff Flake, and South Carolina Sen. Lindsey Graham.